(5) Convergence. Some animals see a set of objects off to the left through
one eye and an entirely different set off to the right through the other. This
characteristic doubles their field of vision and allows them to detect predators behind
them. In humans, both eyes focus on only one set of objects--a characteristic called
single binocular vision. Single binocular vision occurs when light rays from an object
are directed toward corresponding points on the two retinas. When we stare straight
ahead at a distant object, the incoming light rays are aimed directly at both pupils and
are refracted to identical spots on the retina of both eyes. But as we move closer to the
object, our eyes must rotate medially for the light rays from the object to hit the same
points on both retinas. The term convergence refers to this medial movement of the two
eyeballs so they are both directed toward the object being viewed. The nearer the
object, the greater the degree of convergence necessary to maintain single binocular
vision. Convergence is brought about by the coordinated action of the extrinsic eye
muscles.
Section III. THE SPECIAL SENSE OF HEARING (AUDITORY)
1-6.
GENERAL
The human ear serves two major special sensory functions-- hearing (auditory)
and equilibrium (balance). Receptors for sound waves and receptors for equilibrium are
located in the ear. The stimulus for hearing is sound waves, and the stimulus for
1-7.
ANATOMY
The ear is divided into three principal regions: the external (outer) ear, the middle
ear, and the internal (inner ear). See figure 1-6 which shows the anatomy of the ear.
a. The External Ear. The external ear begins on the outside of the head in the
form of a funnel-shaped auricle (pinna). Actually serving as a funnel, the auricle directs
airborne sound waves into the external auditory meatus. The external auditory meatus
is a tubular canal extending into the temporal portion of the skull.
b. The Middle Ear. The middle ear is also called the tympanic cavity. It is a
small epithelial-lined, air-filled cavity hollowed out of the temporal bone. The eardrum
separates this cavity from the external ear. The cavity is separated from the internal ear
by a thin bony partition that contains two small openings: the oval window and the round
window.
(1) Tympanic membrane. At the inner end of the external auditory meatus
is the tympanic membrane. The tympanic membrane (eardrum) is a circular membrane
which separates the external auditory meatus from the middle ear cavity. The tympanic
membrane vibrates (mechanically oscillates) in response to airborne sound waves.
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